PRTX-100 is a highly-purified bacterial protein called Staphylococcal Protein A. In this study, cohorts of patients with active RA will receive sequentially higher doses of PRTX-100. There will be an inactive placebo cohort for comparison. Patients who do not attain low RA disease activity, by a commonly used measure, will leave the study at 3 months after their first dose of study drug.